This is the first in a series of upcoming posts about grazing in prairies – something that is common and widely accepted in western tallgrass prairie and mixed-grass prairies but much less so in eastern tallgrass prairies. Sorry for the length – there’s a lot to say in an introduction to this topic…
Cattle and prairies are like oil and water to some prairie biologists and enthusiasts – especially in the eastern tallgrass prairie region. There’s no doubt that many prairies have suffered from chronic overgrazing during the last couple of centuries. But to broadly categorize grazing as bad for prairies because some have been overgrazed is like saying that food is bad because some people are obese.
It's easy to see how people can be sceptical about the value of grazing if this is their mental image of cows in prairies.
To be clear, I’m not arguing that every prairie needs grazing. Some prairies are too small for grazing to make any sense. In other cases, the owner and/or manager of a prairie may be completely satisfied with the plant composition and habitat availability within a prairie – in which case the current management is probably perfectly adequate.
On the other hand, I think there is room for much more discussion about some potential benefits of grazing in prairies. However, in order for that discussion to proceed, everyone involved needs to come into the discussion with an open mind. Despite arguments about the extent to which bison and other large grazers were abundant in eastern tallgrass prairies – and for how long – there is no question that those prairies have experienced some pretty extensive and intensive grazing at times over the last several thousand years. And they’ve survived. That doesn’t mean we can just dump a load of cattle into a prairie and everything will be peachy. But I think it’s important not to summarily dismiss grazing as irrelevant or automatically dangerous to prairies.
One of the obstacles to open discussion about grazing in prairies is the assumption that if you’ve seen one grazed prairie you’ve seen them all. In reality, grazing may be the most flexible management tool available – you can control the timing, duration, intensity, and frequency of application. Stocking rate (grazing intensity) is probably the most important of those. The number of cattle (or bison or whatever) per unit area makes a tremendous difference in the way the site is grazed. Light stocking rates allow the grazer to be selective – eating only what it wants. In my experience, and that of many other managers, cattle strongly prefer grass over forbs when given the choice. When stocking rates are higher, or when cattle are kept in a prairie long enough that they run short of grass to eat, they eat whatever is available. Many species of plants that have long been considered “ice cream” plants for cattle – plants that cattle will repeatedly graze to the ground until they eventually disappear – are treated much differently by cattle under a light stocking rate. Because the variability in the way grazing can be applied is not often recognized by researchers or casual observers, the world is full of reports and articles that document differences between “grazed” and “ungrazed” prairies. In most cases, the “grazed” prairies are chronically overgrazed – not a fair comparison.
Cattle can be very selective grazers, and under light stocking rates show strong preferences for grass over forbs - even prairie legumes like purple prairie clover that are usually considered to be "ice cream plants".
Another erroneous assumption is that a grazed plant is a dead plant. Being defoliated (clipped off) now and then doesn’t kill prairie plants. If it did, mid-season haying and growing season burns would be devastating to prairies – and they’re not. If a perennial plant is allowed time to recover from being nipped or mowed off, it’ll come back just fine – especially if the competing grasses surrounding it are also weakened by the same defoliation event. Defoliation is only fatal for perennial plants when it occurs continuously, season after season, with no time for recovery. Because of that, it’s easy – but incorrect – to look at a prairie undergoing short-term intensive grazing and assume that many of the prairie plants are gone because they’ve been grazed off. A prairie that looks short and ragged while cattle are present can be tall and flowery again after just a season or two of rest. It’s true that when grazing treatments (or any other prairie management treatment) are applied repetitively, some plant species can gain a repeated advantage over others, limiting plant diversity. In contrast, however, a dynamic (non-repetitive) management regime allows all plant species to complete their life cycle now and then and ensure their continued position in the plant community.
As part of a broader management plan, there are a number of ways periodic grazing can potentially be a valuable tool. For example, intensive grazing in the spring followed by summer rest can repeatedly defoliate and stress cool-season exotic grasses like smooth brome while providing opportunities for many native plants to gain an advantage. Grazing cattle under a light stocking rate can be used to create unique and valuable habitat structure because the cattle clip off some plants but not others. That selective grazing creates patchy vegetation that can help invertebrates and reptiles more easily thermoregulate and provide many animal species the ability to forage in open areas while remaining close to protective cover. Under a higher stocking rate, cattle can create and maintain large patches of short vegetation that benefit bird species like upland sandpipers. After that kind of intensive grazing, many plant species that were previously suppressed by dominant grasses can be temporarily released from that competition.
Selective grazing by cattle can help forbs compete against dominant grasses, but also creates valuble habitat structure that is difficult to obtain with other management tools.
One grazing strategy that has been getting a lot of recent attention is called patch-burn grazing. In its most basic form, patch-burn grazing uses prescribed fire to concentrate grazing in some portions of a prairie while other portions get little to no grazing. The system works because recently burned prairie patches are extremely attractive to cattle and those cattle will spend the majority of their time there – to the exclusion of less recently burned patches. The intensity of grazing in burned patches and the extent to which unburned patches are grazed, both increase with stocking rate. Whenever a new patch of prairie is burned, the grazing shifts to the new burn patch and the previous patch begins its recovery phase. At any one time, a portion of the prairie is being intensively grazed, a portion is recovering from recent grazing, and other portions are largely ungrazed – all without any interior fences.
Researchers from Oklahoma State University and The Nature Conservancy have been testing patch-burn grazing as a way to improve wildlife habitat in ranch country by providing more without heterogeneous vegetation structure without sacrificing income for ranchers. They and others have documented benefits to wildlife species such as prairie chickens and Henslow’s sparrows that require habitat structure not often found in more traditionally-grazed grassland. They’ve also found that prairies under patch-burn grazing support a larger total diversity of wildlife species than more homogenously managed grasslands because of the wider range of available habitat types.
I began experimenting with patch-burn grazing in 2001 because I was looking for a way to maintain plant diversity in restored (reconstructed) prairies along the Platte River in Nebraska. We had invested a lot of time and energy seeding 150-230 plant species into former crop fields and I needed a way to manage them that would prevent grasses from becoming too dominant and/or plant diversity declining over time. I reported some preliminary results of our work in an article for the journal Ecological Restoration in 2005, but now have 9 years of data from our sites. Overall, I’ve been very encouraged by the results I’ve seen. Our restorations are maintaining their plant diversity and species composition – while exclosures that get prescribed fire but not grazing have lower plant diversity and are largely dominated by grasses. I’m also tracking individual plant species and have seen plant species traditionally thought to decrease in abundance under grazing maintain their abundance over 9 years of annual patch-burn grazing.
Cattle grazing in the burned patch of a patch-burn grazed restored prairie (light stocking rate) - note the ungrazed Canada milkvetch (bottom left) and compass plant (tall).
I continue to experiment with ways to modify the basic patch-burn grazing method to favor plant diversity, including the use of varying stocking rates and the length of the grazing season. In addition, most of my sites are relatively small (between 100 and 200 acres in size) so plants that are especially tasty to cattle sometimes get grazed even in unburned areas. It appears that a year of complete rest from grazing now and then is probably important (and sufficient) for maintaining strong populations of those species.
There is still much to learn about the potential usefulness of patch-burn grazing and other methods of cattle grazing for prairie management. For example, most research projects have not been set up in ways that have provided long-term or intensive data on the impacts of grazing on individual plant species over time. Konza prairie researchers have a long history research on grazing – largely on its effects on ecological processes, however (but with insights on plant species responses as well), and some people are concerned about how well their data on shallow-soil prairies applies to other prairie types. While my own data show promising results regarding plant species responses, I’m looking primarily at restored prairies and my sites are on sandy soils with annual rainfall amounts between 25 and 28 inches per year. I’m seeing cattle graze select very strongly for grasses, with very little grazing of forbs – especially with light stocking rates. However, it’s difficult to extrapolate those results to prairies with different soil types and average precipitation without considerably more testing.
However, the questions about how or whether a particular grazing system or another impacts prairies miss a larger point. The bigger question is whether or not grazing itself has potential as a tool for prairie managers to use to address specific challenges on prairies, including encroaching invasive species, over-dominance of native grasses or other plants, homogenous wildlife habitat structure, etc. The answers to those questions will have to come from experimentation by many people at many sites. I hope that prairie biologists, managers, and enthusiasts will continue their legacy of innovation and take an open-minded view toward prescribed grazing. We have too few prairies left, and too many threats to them, not to explore every tool available to us.
(There are a number of other issues related to grazing that I don’t have room to discuss here – but will in later posts. Those include differences between cattle and bison grazing, more details about my research on individual plant reponses to grazing, issues relating to cattle behaviors like trailing and concentrating around water and shade, the infrastructure needed for cattle grazing, and others. Stay tuned – or ask specific questions by commenting below or sending me an email at firstname.lastname@example.org.)